This invention relates to improved methods for providing color negative images from color negative photographic silver halide elements. Thus, this invention relates to the photographic industry and to photochemical processing in particular.
The conventional image-forming process of silver halide photography includes imagewise exposure of a color photographic silver halide recording material to acetonic radiation (such as visible light), and the eventual manifestation of a useable image by wet photochemical processing of that exposed material. A fundamental step of photochemical processing is the treatment of the material with one or more developing agents to reduce silver halide to silver metal. A useful color image consists of one or more images in organic dyes produced from an oxidized developing agent formed wherever silver halide is reduced to metallic silver.
To obtain useful color images, it is usually necessary to remove all of the silver from the photographic element after color development. This is sometimes known as xe2x80x9cdesilveringxe2x80x9d. Removal of silver is generally accomplished by oxidizing the metallic silver, and then dissolving it and undeveloped silver halide with a xe2x80x9csolventxe2x80x9d or fixing agent in what is known as a fixing step. Oxidation is achieved with an oxidizing agent, commonly known as a bleaching agent.
Color photographic elements can be designed to provide either color negative or color positive images. For example, color negative images can be produced by imaging and appropriate color processing of imagewise exposed color negative films. The typical commercial processing methods for such films generally include color development, bleaching, fixing and final rinsing or stabilizing steps (for example, the conventional Process C-41 method commercialized by Eastman Kodak Company).
The most common bleaching agents for color photographic processing are complexes of ferric ion and various organic chelating ligands (such as aminopolycarboxylic acids), of which there are hundreds of possibilities, all with varying bleaching activities and biodegradability. Common organic chelating ligands used as part of bleaching agents for color negative film processing include ethylenediaminetetraacetic acid (EDTA), 1,3-propylenediaminetetraacetic acid (PDTA) and nitrilotriacetic acid (NTA).
Ferric complexes with PDTA or its salts are excellent bleaching agents can be used in processing color negative films in the commercial Process C-41 processing method.
The bleaching agents tend to form thick, gelatinous precipitates in the presence of inorganic phosphate ions. Such ions are common in various photoprocessing compositions, and may be present in the color developing compositions that are used in processing color negative materials. Phosphate ions in these compositions are apparently carried over into the bleaching solution. If the PDTA-based bleaching agents are present, the undesirable precipitates are formed.
One way to solve this problem is to remove the phosphate ions from the various processing compositions so carryover solutions do not contaminate the bleaching composition. It has not been possible to remove sufficient inorganic phosphate ions from all of those compositions.
Thus, there remains a need for PDTA bleaching compositions that are free of precipitates in the presence of inorganic phosphate ions.
The problems with known photographic photoprocessing compositions and methods are overcome with the use of a method of photographic processing comprising bleaching an imagewise exposed and color developed color negative photographic silver halide element with a photographic bleaching composition or a photographic bleach-fixing composition that in aqueous form has a pH of at least 4 and comprises:
a) at least 0.01 mol/l of a bleaching agent that comprises ferric ion chelated with 1,3-propylenediaminetetraacetic acid or a salt thereof, and
b) at least 0.005 mol/l of an organic polyphosphonic acid, an aminopolysuccinic acid, or a polycarboxylic acid containing at least one hydroxy group as a compound to prevent bleaching agent precipitation.
The advantages of this invention are several. The color negative photographic elements can be processed using a highly effective and inexpensive photographic bleaching agent without the problem of precipitation if inorganic phosphate ions are present. Because a more effective bleaching agent can be used, less of it is discharged to the environment in effluent. Moreover, other components in the bleaching or bleach-fixing composition may be lessened, thereby further lowering costs and environmental impact.
The precipitate problem is overcome by having certain bleaching agent precipitation preventing compounds in the bleaching or bleach-fixing composition. These compounds can be introduced directly into the bleaching or bleach-fixing composition (for example during manufacturing or use as a separate liquid or solid additive), or indirectly by carryover from prior processing solutions, such as a photographic color developing solution. Generally, the bleaching agent precipitation preventing compounds used in the practice of this invention are present in the bleaching or bleach-fixing composition in an amount of at least 50% of the concentration of the inorganic phosphate ions that can be 0.0005 mol/l or more in that composition. Phosphate ions can be introduced directly or indirectly into the bleaching or bleach-fixing composition as well as the bleaching agent precipitation prevention compounds.